1. Field of the Invention
The invention is directed to a method and apparatus for non-destructive testing of materials, and, in particular, for the determination of the thickness of plastics, as well as of bonding and coating materials, by means of high frequency electrical or magnetic alternating fields. The principal area of application of the invention is the testing of plastic components, as well as the determination and monitoring of the thickness of coatings and bonding layers. The invention is however not limited to this area only, rather it can be used in any other sphere, in which the known thickness measuring methods cannot lead to satisfactory results because of reasons described below.
2. Description of the Prior Art
Various techniques are known for determining the thickness of plastic parts, coatings and bonding layers, which techniques differ depending upon the type of part or material to be tested, and which lead to different results depending upon the purpose of the tests.
A. Testing of Plastic Parts
In the fabrication of plastic parts, particularly of endless reel parts such as pipes, foils and the like, variations in the thickness thereof occur because of non-uniformities in the production process. These variations in thickness are difficult to measure non-destructively, so that plastic components (for instance, shopping bags) are greatly limited in their overall strength because of the existing weak spots.
B. Testing of Coatings on Accessible Sides
1. Optical Methods:
These can be utilized for thickness testing of transparent coatings and bonding layers applied to metallic or plastic materials. They are based either upon the detection and evaluation of the interferences produced or upon an evaluation of the optical effect of a concentration of the coating or bonding material with optically visible particles. The thickness of the layer can indeed be largely determined with these optical methods, however the scope of their utilization is very limited, since they are only usable for transparent materials of very thin layers.
2. Test by Means of Measuring Probes:
These methods are based upon placing a measuring probe upon the layer whose thickness is to be determined, and measuring and evaluating the current or voltage profile of the field of the probe for determination of the distance between measuring probe and the base material, from which one can deduce the thickness of the layer applied to the base material. This method is, however, only utilized for coating materials which are non-conductive or do not exhibit any magnetic properties. A prerequisite for the utilization of these processes is that the coating must be cured prior to measurement, because otherwise, the probes could not be placed upon it. Furthermore, the coating must be applied to an accessible side or spot. If this is not the case, which is entirely possible in the automatic coating and production processes presently being used, these methods fail completely.
C. Testing Coatings on Inaccessible Sides
If the coating has been applied to a side which is inaccessible, then information about the thickness of the coating can only be acquired by enriching of the adhesive with radioactive particles or heavy metals and subsequently measuring the radioactive emissions. These methods, however, require not only considerable expense but also special protection measures. Therefore, they cannot be used in many cases. The same applies for measurement by means of neutron beams. In this way, the thickness of a coating from organic materials can indeed be determined based on electrically and magnetically conducting components, however the appropriate measures (relatively high neutron flow, protection) are relatively costly. Apart from that, the measuring accuracy is also impaired if the composition (material thickness) of the base material located beneath the coating, is not exactly known.